1. Brief Description of the Invention
A method has been developed for the desulfurization of a liquid feeds containing organosulfur compounds using metallic sodium dissolved in liquid ammonia at a much lower temperature than previously thought possible.
2. Related Art
Recent regulations imposed by the Environmental Protection Agency have mandated the reduction in sulfur for on-road diesel fuel to 15 ppm S in the U.S. by June 2006; and refiners are currently installing technologies to meet this target. However, because on-road diesel fuel will be distributed through pipelines to customers across the country and because those pipelines will also be used to transport jet fuel and kerosene, which are substantially higher in sulfur, refiners will need to produce on-road diesel fuels having 6-7 ppm S. Even then, transport through the pipelines is expected to produce off-spec diesel (16-75 ppm S) as a result of mixing at the interfaces between the diesel and jet or kerosene. This off-spec diesel can be downgraded to the next lower quality fuel for the next four years resulting in a financial loss to the refiner. After 2010, no downgrades will be allowed. Therefore, there is a need for a process that can easily remove sulfur from distillate fuels, at the end of the pipeline or in refinery environments that can help refiners meet their low sulfur targets economically.
The literature describes several ways of removing sulfur from petroleum streams, which fall into two primary categories: (a) catalytic hydrotreating processes in which the feed is passed over a hydrotreating catalyst at elevated temperatures and hydrogen pressures and (b) non-hydrotreating processes. The present invention is an improved non-hydrotreating process.
Non-hydrotreating processes can be further divided into those that remove the entire sulfur-bearing molecule from the feed by adsorption onto a selective adsorbent, as exemplified by Khare (U.S. Pat. No. 6,274,533; U.S. Pat. No. 6,338,794 and U.S. Pat. No. 6,482,314), and those that remove only the sulfur by chemical reaction. The latter can further be divided into those that are oxidative in nature, as exemplified by Rappas (U.S. Pat. No. 6,402,940), Ohsol, et al (U.S. Pat. No. 5,985,137; U.S. Pat. No. 5,948,242), Yen (U.S. Pat. No. 6,402,939) and Gunnerman (U.S. Pat. No. 6,500,219), and those that are reducing in nature. The most important of the reducing processes is reaction of a feed containing organosulfur species with elemental sodium as exemplified by Brons, et al (U.S. Pat. No. 6,210,564), Baird (U.S. Pat. No. 4,003,824; U.S. Pat. No. 4,123,350) and Bearden (U.S. Pat. No. 3,787,315; U.S. Pat. No. 3,788,978; U.S. Pat. No. 3,791,966; U.S. Pat. No. 3,976,559 and U.S. Pat. No. 4,076,613).
In order to contact elemental sodium with feed, the sodium is typically melted (m.p.=97.8° C.) and added to the feed as a dispersion of small droplets. Even with intense mixing, it is not possible to produce extremely small droplet sizes. As a result, sodium on the surface of the droplets may react with sulfur in the feed to form a skin of sodium sulfide (Na2S). This skin remains on the droplet exterior surface, and as Na2S has a melting point of 1,180° C., it remains as a solid under the conditions of desulfurization. The skin thereby inhibits further reaction between the sodium in the interior of the droplet and sulfur in the feed. For these reasons, it has been necessary in the prior art to use substantially higher ratios of sodium to feed sulfur than are stoichiometrically required in order to remove sulfur to the desired level.
Other methods to incorporate sodium include solvating sodium with ammonia, mixing with the feed stream and then reducing the pressure of the resulting liquid below the vapor pressure of ammonia to vaporize the ammonia solvent. This is disclosed in U.S. patent application Ser. No. 10/826,621, hereby incorporated by reference. This method is more economical and efficient than previous methods.
3. Summary of the Invention
In accordance with the present invention, a method of removing sulfur from liquid feeds is presented which overcomes limitations in the prior art. This invention is a method of removing sulfur from a liquid feed containing organosulfur compounds. The method, in the preferred embodiment, comprises the steps of:                (a) dissolving metallic sodium in a solvent to form a solution of sodium atoms;        (b) combining the resulting liquid solution of sodium with (1) a liquid hydrocarbon feed comprising organosulfur species and as an option, (2) a liquid hydrogen donor solvent        to form a combined stream at a temperature of addition and at a pressure above the vapor pressure of the solvent at the temperature of addition;        (c) passing the feed to a pressurized reactor vessel;        (e) reacting the feed and sodium for sufficient time and at sufficient temperature to form a modified composition comprising one or more sulfur-containing species and less of the organosulfur species than had been present in the hydrocarbon feed;        (f) extracting the sulfur-containing products from the modified composition using an extraction fluid.        
It is readily apparent to those skilled in the art that many different solvents, including but not limited to ammonia and some ethers, may be used to dissolve metallic sodium and that many different reactor and flow configurations may be used to carry out the desulfurization process.